Changes which occur in the lattice parameter of a neutron irradiated copper-boron alloy during the early stages of annealing, have been measured. After irradiation at 100° C, the lattice parameter is larger than that of the unirradiated alloy. During annealing, the lattice parameter decreases until it becomes smaller than that of the unirradiated alloy, and then increases. After complete annealing, the lattice parameter is equal to that of the original untreated alloy. Initially the helium atoms, which are formed by an (n, α) reaction from B10, occupy interstitial positions. During annealing, each helium atom captures two vacancies and enters substitutional solution, before migrating to form gas bubbles. This process causes the lattice to contract. As bubbles form, helium leaves solution and the concentration of heliumvacancy complexes is reduced. There is a corresponding expansion of the lattice parameter. The activation energy for the first stage is 0.76 ± 0.1 eV. The activation energy for the second stage is 0.96 ± 0.1 eV, which is possibly the energy required to move a helium atom-vacancy complex through the lattice.